1. Field of the Invention
The present invention relates to a dispersion-compensating optical fiber and an optical transmission path. The present specification is based on patent applications filed in Japan (Japanese Patent Application No. 2000-54646, Japanese Patent Application No. 2000-159071, Japanese Patent Application No. 2000-216587, Japanese Patent Application No. 2000-241547 and Japanese Patent Application No. 2000-266169), and the contents described in said Japanese patent applications are incorporated as a part of the present specification.
2. Background Art
Optical communication systems that transmit at the 1.55 xcexcm wavelength band (the so-called C-band: typically covering a range of about 1.53-1.57 xcexcm) are used practically by combining single-mode optical fibers for transmission such as a xe2x80x9c1.3 xcexcm band dispersion single-mode optical fiber having nearly zero chromatic dispersion at a wavelength of 1.3 xcexcmxe2x80x9d or a xe2x80x9cstandard single-mode optical fiberxe2x80x9d, and dispersion-compensating optical fibers.
For example, since the chromatic dispersion of a 1.3 xcexcm single-mode optical fiber is roughly +17 ps/nm/km (positive chromatic dispersion) at a wavelength of 1.55 xcexcm, when this is used to perform optical communications in the 1.55 xcexcm wavelength band, considerable chromatic dispersion occurs. In contrast, since dispersion-compensating optical fibers have negative chromatic dispersion in which the absolute value in the 1.55 xcexcm wavelength band is comparatively large, by combining these in the manner described above, the chromatic dispersion that occurs in ordinary 1.3 xcexcm single-mode optical fibers extending, for example, for several tens of kilometers, can be compensated by a dispersion-compensating optical fiber having a comparatively short used length.
In addition, since the dispersion slope of a 1.3 xcexcm single-mode optical fiber in the 1.55 xcexcm wavelength band is roughly +0.06 ps/nm2/km (positive value), in order to compensate according to this dispersion slope with the chromatic dispersion, it is preferable to use a dispersion-compensating optical fiber having a negative dispersion slope. If dispersion slope can be compensated, this can also be used in applications involving transmission of a plurality of pulsed light having different wavelengths as in the manner of high-density wavelength division multiplexing transmission (DWDM transmission).
On the other hand, transmission characteristics deteriorate when non-linear effects occur in optical fibers. In the case of propagating high-power signal light in the manner of optical communication systems using wavelength division multiplexing transmission and an optical amplifier that have already been used practically in particular, due to the high power density, non-linear effects tend to occur easily, resulting in the need for technology that suppress non-linear effects.
Although methods for suppressing non-linear effects have been proposed, including a method in which the non-linear refractive index of the optical fiber is decreased by reducing the amount of Ge, F or other dopant doped to the core, and a method in which Brillouin scattering, which is one of the non-linear effects, is suppressed by changing the outer diameter of the optical fiber when drawing from the fiber base material, enlargement of the effective area (which may be abbreviated as Aeff) of the optical fiber is a particularly effective method.
However, in the above-mentioned dispersion-compensating optical fibers of the prior art, although such optical fibers have been developed which attempt to improve the so-called performance index (FOM; Figure of Merit), which indicates the amount of chromatic dispersion per unit loss, while also being able to compensate the dispersion slope, it has been difficult to simultaneously realize these characteristics along with enlargement of Aeff.
In addition, a system that performs optical communication in the so-called L-band (1.57-1.63 xcexcm), which is of a longer wavelength than the C-band that has been used in the past, has recently been examined. Wavelength bands used for optical transmission at present or in the future are reaching or will reach a broad range of 1.45-1.63 xcexcm, which includes the so-called S-band (1.45-1.53 xcexcm).
Thus, although a dispersion-compensating optical fiber is required that is able to compensate the chromatic dispersion and dispersion slope of single-mode optical fibers for transmission in not only the C-band, but also other wavelength bands such as the S-band and L-band, conventional dispersion-compensating optical fibers have been unable to adequately satisfy this requirement. Consequently, these optical fibers have been inadequate particularly in applications to wavelength multiplexing, high-speed, long-distance transmission and so forth.
In addition, although large Aeff is also simultaneously required in these wavelength bands, conventional dispersion-compensating optical fibers were unable to accommodate this requirement as well.
Moreover, dispersion-compensated optical fibers are required to have single-mode propagation at the used wavelength band and bending loss that is small enough to allow practical use.
In addition, these dispersion-compensating optical fibers of the dispersion slope-compensating type have conventionally been incorporated in optical communication systems in the form of modules by being housed in, for example, a suitable case. Recently however, studies have been conducted that attempt to form the dispersion-compensating optical fiber itself into a cable and insert it into the transmission path, and several of these attempts have been reported.
This is because, if a dispersion-compensating optical fiber itself was able to be used as a transmission path, since it would be possible to eliminate the arranging space of the module, while also being able to substantially shorten the length of the optical fiber through which the optical signals are transmitted, the transmission characteristics of the overall system could be improved.
However, although reports have been made regarding dispersion-compensating optical fibers of the dispersion slope-compensating type that place the emphasis on compensation of chromatic dispersion and dispersion slope as well as reduction of loss, there have been no effective studies or reports made regarding enlargement of Aeff.
Suppression of non-linear effects is essential for achieving the faster speeds, longer distances and wavelength multiplexing described above, and in the case of inserting a dispersion-compensating optical fiber of the dispersion slope-compensating type in the form of a transmission path, there are cases in which it is difficult to achieve practical use unless Aeff is provided to an extent that is able to effectively suppress non-linear effects.
In addition, dispersion-compensating optical fibers of the prior art required that, for example, the refractive index of the core center be larger than that of an ordinary single-mode optical fiber, and had the problem of increasing the amount of dopant doped to the core center.
Normally, a core center is formed from quartz glass doped with a dopant such as germanium etc. that provides the action of increasing refractive index, while the cladding provided around an outer periphery of the core is formed from pure quartz glass or fluorine-doped quartz glass, etc.
In addition, the glass transition point of the quartz glass decreases proportional to the amount of dopant doped. Thus, if the amount of dopant doped increases, since the difference in viscosities of the core and cladding increases when the fiber base material is heated and melted to draw the optical fiber, the drawing rate and drawing temperature are restricted from the viewpoint of mechanical strength, thereby resulting in the problem of being unable to obtain a dispersion-compensating optical fiber with low loss.
In addition, if the refractive index of the core center is high, Aeff tends to decrease. This results in greater susceptibility to the occurrence of non-linear effects, thereby leading to the problem of deterioration of transmission characteristics.
However, in the dispersion-compensating optical fibers of the prior art, optical fibers realizing chromatic dispersion and dispersion slope compensating effects while enabling the refractive index of the core center to be comparatively small have been unable to be obtained, resulting in problems in terms of ease of production, low loss and non-linearity, etc.
In addition, if Aeff is enlarged in an example of a dispersion-compensating optical fiber of the prior art, the absolute value of chromatic dispersion tends to become smaller, resulting in the problem of the length for compensating single-mode optical fibers for transmission becoming excessively long.
The object of the present invention is to provide a dispersion-compensating optical fiber that can be applied to a wide wavelength band, has a large Aeff, and as a result, is resistant to the occurrence of non-linear effects.
Moreover, the object of the present invention is to achieve at least one of the following first through fifth objects after having achieved this object.
A first object of the present invention is to provide a dispersion-compensating optical fiber that is able to guarantee chromatic dispersion of a 1.3 xcexcm single-mode optical fiber over the entire wavelength range of 1.53-1.63 xcexcm, while also being able to guarantee single-mode propagation and have small bending loss.
Moreover, another object is to provide a dispersion-compensating optical fiber that is able to simultaneously compensate dispersion slope.
Moreover, another object is to provide a dispersion-compensating optical fiber having a large Aeff that is able to suppress non-linear effects.
A second object of the present invention is to provide a dispersion-compensating optical fiber of the dispersion slope-compensating type that is able to realize low loss and low non-linearity while maintaining the inherent function of compensating chromatic dispersion and dispersion slope.
More specifically, the object is to provide a dispersion-compensating optical fiber provided with a large Aeff in order to realize low non-linearity.
A third object of the present invention is to provide a dispersion-compensating optical fiber having large Aeff and low loss.
In addition, an object is to provide a dispersion-compensating optical fiber for which the difference in viscosities between the core and cladding during drawing is small.
More specifically, the object is to provide a dispersion-compensating optical fiber that is able to make the relative refractive index difference based on the cladding of the highest layer of the core comparatively small, while also decreasing the amount of dopant doped to this layer.
A fourth object of the present invention is to provide a technology for obtaining a dispersion-compensating optical fiber of the dispersion slope-compensating type having low loss. In addition, an object is to lower bending loss independent of the used wavelength band, without significantly impairing other characteristics, in comparison with dispersion-compensating optical fibers of the prior art.
Moreover, an object is to realize a dispersion-compensating optical fiber suitable for long-distance transmission that is able to suppress the chromatic dispersion value and dispersion slope value at a specific length.
A fifth object of the present invention is to provide a dispersion-compensating optical fiber that is able to compensate chromatic dispersion of a single-mode optical fiber for transmission over a wide wavelength band.
In addition, an object is to provide a dispersion-compensating optical fiber that is able to enlarge Aeff and suppress non-linear effects.
Moreover, an object is to provide a dispersion-compensating optical fiber in which the length required for compensating single-mode optical fibers for transmission, while preventing the absolute value of chromatic dispersion from becoming small even if Aeff is enlarged, is comparatively short.
In order to achieve the above first object, the dispersion-compensating optical fiber of a first embodiment of the present invention is a dispersion-compensating optical fiber that compensates chromatic dispersion of a 1.3 xcexcm single-mode optical fiber over the entire wavelength range of 1.53-1.63 xcexcm wherein, chromatic dispersion at a wavelength of 1.55 xcexcm is xe2x88x9250 ps/nm/km or less, the dispersion slope is negative over the entire wavelength range of 1.53-1.63 xcexcm, a cutoff wavelength is provided at which there is substantially single-mode propagation over the entire wavelength range of 1.53-1.63 xcexcm, bending loss is 30 dB/m or less over the entire wavelength range of 1.53-1.63 xcexcm, Aeff is 20 xcexcm2 or more over the entire wavelength range of 1.53-1.63 xcexcm, and the absolute value of chromatic dispersion during compensation of the chromatic dispersion of a 1.3 xcexcm single-mode optical fiber serving as the target of compensation is 0.5 ps/nm/km or less over the entire wavelength range of 1.53-1.63 xcexcm.
In addition, in order to achieve the above second object, the dispersion-compensating optical fiber of a second embodiment of the present invention characterized in that, in a used wavelength band selected from 1.53 to 1.63 xcexcm, Aeff is 30 xcexcm2 or more, bending loss is 40 dB/m or less, chromatic dispersion is xe2x88x9240 to xe2x88x9210 ps/nm/km, the absolute value of chromatic dispersion over the entire transmission path connected to a single-mode optical fiber for transmission provided with positive chromatic dispersion is 4.0 ps/nm/km or less, the absolute value of dispersion slope over the entire transmission path is 0.03 ps/nm2/km or less, and a cutoff wavelength is provided that allows substantially single-mode propagation at the used length in the above transmission path.
In order to achieve the above third object, the dispersion-compensating optical fiber of a third embodiment of the present invention is characterized in that, a core and a cladding provided around an outer periphery of said cladding are provided,
said core is provided with a central core portion having a refractive index higher than said cladding, an intermediate core portion provided around an outer periphery of said central core portion having a refractive index lower than said cladding, and a ring core portion provided around an outer periphery of said intermediate core portion having a refractive index higher than said cladding,
when radii and relative refractive index differences based on the cladding of the central core portion, the intermediate core portion and the ring core portion are expressed as (a,xcex941), (b,xcex942) and (c,xcex943), respectively,
a is 2-3 xcexcm, xcex941 is 0.9 to 1.5%, xcex942 is xe2x88x920.30 to xe2x88x920.45%, xcex943 is 0.2 to 1.2%, b/a is 2.0 to 3.5, and c/a is 3.0 to 5.0,
in a used wavelength band selected from 1.53 to 1.63 xcexcm, Aeff is 20 xcexcm2 or more, bending loss is 40 dB/m or less, chromatic dispersion is xe2x88x9265 to xe2x88x9245 ps/nm/km, and a cutoff wavelength is provided that allows substantially single-mode propagation, and
the compensation rate of dispersion slope when compensating a single-mode optical fiber, at a length at which chromatic dispersion of said single-mode optical fiber having a zero dispersion wavelength at a wavelength shorter than the above used wavelength band can be compensated to zero, is 80-120%.
In order to achieve the above fourth object, the dispersion-compensating optical fiber of a fourth embodiment of the present invention is characterized in that,
a core and cladding provided around an outer periphery of said core are provided,
said core is provided with a central core portion having a refractive index higher than said cladding, an intermediate core portion provided around an outer periphery of said central core portion having a refractive index lower than said cladding, a ring core portion provided around an outer periphery of said intermediate core portion having a refractive index higher than said cladding, and a side ring core portion provided around an outer periphery of said ring core portion having a refractive index lower than said cladding,
in a used wavelength band selected from 1.45 to 1.63 xcexcm, chromatic dispersion is xe2x88x9270 to xe2x88x9245 ps/nm/km, chromatic dispersion slope is negative, Aeff is 20 xcexcm2 or more, and a cutoff wavelength is provided that allows substantially single-mode propagation, and when a single-mode optical fiber is compensated at a length at which chromatic dispersion of this single-mode optical fiber having zero dispersion at a wavelength shorter than said used wavelength band can be compensated to zero, the compensation rate of dispersion slope defined as RDS(DCF)/RDS (single-mode optical fiber)xc3x97100, when the value obtained by dividing the dispersion slope of the single-mode optical fiber by chromatic dispersion of the single-mode optical fiber is taken to be RDS (single-mode optical fiber), and the value obtained by dividing the dispersion slope of the dispersion-compensating optical fiber by chromatic dispersion of the dispersion-compensating optical fiber is taken to be RDS (DCF), is 80-120%, and bending loss at a wavelength of 1.63 xcexcm is 50 dB/m or less.
Furthermore, RDS is the abbreviation of relative dispersion slope.
In order to achieve the above fifth object, the dispersion-compensating optical fiber as claimed in a fifth embodiment of the present invention is characterized in that, a core and a cladding provided around an outer periphery of said core are provided,
said core is provided with a central core portion having a refractive index higher than said cladding, an intermediate core portion provided around an outer periphery of said central core portion having a refractive index lower than said cladding, and a ring core portion provided around an outer periphery of said intermediate core portion having a refractive index higher than said cladding,
at a wavelength of 1.55 xcexcm, chromatic dispersion is xe2x88x9240 ps/nm/km or less and xe2x88x9265 ps/nm/km or more, chromatic dispersion slope is negative, Aeff is 18 xcexcm2 or more, bending loss is 50 dB/m or less, and the cutoff wavelength allows substantially single-mode propagation, and
the chromatic dispersion of a hybrid transmission line combined with a single-mode optical fiber for transmission for which in 1.55 xcexcm, Aeff is 40 xcexcm2 or more, chromatic dispersion is positive, and the cutoff wavelength allows substantially single-mode propagation is xe2x88x920.5 ps/nm/km or more and +0.5 ps/nm/km or less at a used wavelength band over a continuous range of 0.06 xcexcm or more selected from a wavelength range of 1.45-1.63 xcexcm.
In the first embodiment of the present invention, a dispersion-compensating optical fiber can be provided that is able to compensate chromatic dispersion and dispersion slope of a 1.3 xcexcm single-mode optical fiber over the entire range of 1.53-1.63 xcexcm and compensate single-mode propagation while having low bending loss, large Aeff and is able to suppress non-linear effects.
In the second embodiment of the present invention, a dispersion-compensating optical fiber of the dispersion slope-compensating type can be provided that is able to realize low loss and low non-linearity while maintaining its inherent function of compensating chromatic dispersion and dispersion slope.
In the third embodiment of the present invention, a dispersion-compensating optical fiber having low loss can be obtained by drawing at lower tension than that of the prior art to since the relative refractive index difference of a layer provided with the highest refractive index of a core is small, and non-linear effects can be Suppressed by enlarging Aeff.
In addition, the dispersion-compensating optical fiber of a third embodiment of the present invention is able to construct a hybrid transmission line suitable for wavelength division multiplexing transmission, long-distance transmission and so forth by combining with a single-mode optical fiber.
The dispersion-compensating optical fiber of the fourth embodiment of the present invention is able to reduce the difference in softening temperature and hardening temperature between a central core portion and a cladding, and reduce the difference in viscosity at its drawing temperature during drawing by forming the entire portion, and particularly the cladding, from quartz glass containing dopant.
As a result, the stress remaining in the central core portion and so forth after drawing can be reduced, and even if drawn at a temperature at which practical mechanical strength is obtained, deterioration of transmission loss can be reduced, making it possible to provide a dispersion-compensating optical fiber of the dispersion slope-compensating type, while also having low loss.
The dispersion-compensating optical fiber of the fifth embodiment of the present invention is able to compensate chromatic dispersion of a single-mode optical fiber for transmission over a wide wavelength band, while also being able to enlarge Aeff and suppress non-linear effects. Accordingly, a hybrid transmission line can be provided that is suitable for wavelength division multiplexing transmission and long-distance, high-speed transmission.
In addition, since the absolute value of chromatic dispersion is not reduced excessively even if Aeff is enlarged, chromatic dispersion can be compensated of a single-mode optical fiber for transmission at a comparatively short used length.